Magnetic nanoparticles have become a relevant object of research in the last few decades. They have a great variety of applications, including information storage, catalysis or biomedicine. In this particular field, iron oxides (either magnetite, Fe3O4 or maghemite, γ-Fe2O3) are typical materials used, since they are the only magnetic materials approved for their use in humans by the Food and Drug Administration.
Although spherical nanoparticles have traditionally caught the attention of researchers, scientists are starting to look to anisotropic structures. Nanocubes, for instance, have a larger surface than spheres for the same volume, which may be advantageous for certain applications.
In brief, the Magnetic Nanostructures Group has designed a new fast and reproducible synthesis pathway to grow pure, highly crystalline Fe3O4 nanocubes in a broad size range, from 9 to 80 nm in edge length, with excellent particle size distribution. The possibility to adjust the size with nanometric control eases the optimisation for any intended applications. This research, whose first author is the ICN2 PhD student Javier Muro-Cruces, counted with the collaboration of the Instituto de Nanociencia, Nanotecnología y Materiales Moleculares (Toledo), the Università di Pisa, the IN2UB, the Università degli studi di Firenze and ICCOM-CNR (Florence). The results have been published in ACS Nano.
The magnetic properties of the nanoparticles are strongly affected by the particle size. Indeed, the most interesting sizes for biomedical applications lie around the transition from ferrimagnetism to superparamagnetism (≈15 nm). However, when particle size is reduced, there is an increase of the surface-to-volume ratio, involving a rise in the surface energy making anisotropic structures energetically less favourable. Therefore, any perturbation during the growth stage will tend to drive the growth towards either less anisotropic structures (i.e., spheroidal particles) or larger sizes. In the proposed new method, to favour the cubic morphology at the nanoscale, the use of a surfactant system composed of sodium oleate and oleic acid and a mixture of three solvents (1-octadeceen, benzyl ether and 1-tetradecene) is used to kinetically control the growth of the nanocubes and keep the reaction temperature constant at reflux (avoiding the instabilities related to the use of benzyl ether alone).
In this work, the researchers also have shown that nanocubes exhibit a better performance than spherical nanoparticles in magnetic hyperthermia (using heat to treat tumours) and as contrast agent (used in magnetic resonance imaging). Besides, their chemical approach is versatile and thus has been expanded to other materials, such as cobalt and manganese ferrites and Mn3O4. In a nutshell, the quality and versatility of this synthesis route make it a relevant approach to deepen in the studies of nanocubes and their multiple applications.